Capacitor microphones in which a permanently polarized dielectric, or electret, is used to obviate the need for an external high-voltage supply are well known in the art. Because of their small size, relative insensitivity to temperature or humidity changes, and extended high-frequency response, they are especially suited for use in citizen's band transceivers, military communications systems, and the like. Because of their relatively low output, however, electret microphones require a local preamplifier circuit which must be well shielded to avoid external electromagnetic interference.
Fraim et al U.S. Pat. No. 3,816,671 describes an electret microphone assembly in which a first insulating support disposed between the front and back walls of a conductive housing carries the back electrode of the electret microphone, while a second insulating support forming the back wall of the housing carries a preamplifier circuit on the front or inside surface thereof. The outside surface of the second support carries the voltage supply and output terminals of the preamplifier and is metallized over most of the remainder of the surface area to provide electrostatic shielding.
While the patentees claim to be successful in accomplishing the objects of their invention, their microphone assembly does have some drawbacks. The use of separate supports to support the electromechanical and electronic components, respectively, adds to the complexity and expense of their overall assembly. Further, the use of a metallized insulating support both as an amplifier support and as a back wall of the housing exposes the amplifier to physical shock and to electromagnetic interference. The use of the back wall as an amplifier support also makes it impractical to provide a rear port to render the microphone responsive to velocity rather than pressure when directionality is desired.
In another type of electret microphone assembly known to the art, a single insulating support disposed between the back and the front of the conductive housing carries the back electrode of the electret microphone on its front surface and carries a preamplifier circuit on its rear surface. A coaxial cable entering the housing through an aperture formed in the back wall thereof has its inner conductor coupled to the preamplifier output to provide an external connection. While this arrangement is somewhat simpler than the arrangement disclosed by Fraim et al and better shields the amplifier circuit, it likewise rules out the provision of a rear port to give the microphone a directional response characterisic.